Known in the art is a device for making composite copies of cut diffraction gratings, comprising an illuminating system and, arranged one after another in the direction of the luminous radiation, a collimating system, a transparent cut grating composed of a substrate with rulings cut therein, a substrate of the copy grating and two reflecting cut diffraction gratings which are master gratings facing the copy substrate by their rulings, the transparent grating, as well as the substrate of the copy grating and the illuminating system being secured stationary in relation to other master gratings adapted to be displaced (cf., for example, F. A. Mitin, A. M. Nizhin, G. N. Rassudova: Composite Copies of Measuring Diffraction Gratings, in Optiko-Mekhanicheskaya Promyshlennost, in Russian, 1975, No. 9, pp. 47-50).
This device makes use of the contact duplication process and operates as follows. Two master gratings are placed on the substrate of the copy grating and adjusted by means of the interference moire bands which can be seen at the place of contact of the master gratings. Then the master gratings are removed, resin is poured over the substrate, and master gratings are again very carefully placed on the substrate, pressed against this substrate by means of spring supports. The arrangement is checked by the moire band pattern. Additional adjustment is performed if necessary during the 15-20 minute period until gel formation is completed. After bonding, in about two hours, the excess of resin is removed, and in 18-20 hours the copy hologram is separated from the master gratings, washed in distilled water to get rid of the chemical substances used in the copying process. This device can be used to produce composite copies comprising any number of sections. To this end, one of the master gratings is separated from the two-section copy which has two master gratings bonded thereto and bonded to the free space on the substrate at the end of the second master grating. In this manner composite copies can be made and the number of rulings can reach 1,200 for 1 mm.
But this device is deficient in that the relative position of the master gratings being copied can be performed with an accuracy of a/10, where a is the grating constant or the slit spacing period. The mechanical wear of the master gratings is the reason of the gradual deterioration of this accuracy.
In addition, the device offers no means to trim the edge of the master gratings exactly parallel to the rulings and with an accuracy of one slit period. There are portions which are not ruled at all. The surface of these portions is prominent in relation to the rest of the grating, producing so called "ladders".
Even minute "ladders" are inadmissible in high-quality spectroscopic composite copies, and they are bound to appear due to inaccurate adjustment of the surfaces of the master gratings in one plane.
Moreover, the adjusted master gratings have to be separated from the surface of the copy grating to pour the resin over its surface. So the master gratings are separated from the copy grating and then reinstalled, the adjustment is done again. The technological process, therefore, becomes far too complicated.
One more disadvantage consists in the slow polymerization of resin, which lasts for about 20 hours and makes the copying process too lengthy. In case a large number of copies is to be obtained within this period, the number of copies should be equal to the number of master gratings, which is economically inefficient. In addition, mechanical treatment of the original grating is detrimental to its quality.
It is common knowledge that holography gratings are superior in many respects to cut diffraction gratings. Their frequency can reach 6,000 lines/mm in the optical range, the size being the same.
Master holography gratings are recorded by means of a contactless interference technique. A larger frequency can be put into gratings of the same length. The length and frequency of cut diffraction gratings is limited in principle by the geometrical dimensions of a cutting tool, its wear during the cutting process and impossibility to replace the tool during this process.
The use of a coherent radiation source for interference copying results in less stringent requirements to the size of the gap between the master hologram and its copy. In case of incoherent illumination, this gap should be of the order of the grating pitch. Thus, when the frequency of the grating is 1,000 lines/mm, the grating pitch is 1 micrometer. A gap of 1 micrometer is impossible to maintain, particularly when the substrate is long enough. With the coherent radiation the gap can be several hundreds of microns and more. However, the gap should not be widened excessively up to several centimeters between the master hologram and its copy, since double images can be produced - two virtual and two real ones (cf., D. B. Brumm, Double Images in Copy Holograms, Applied Optics, 1967, v. 6, No. 3, pp. 588-589).
Known in the art is a device for copying hologram gratings, comprising a coherent radiation source and, arranged one after another in the direction of the coherent radiation, a collimating system, a master hologram grating composed of a substrate and a recording layer, and a copy grating which is a substrate with a recording layer, the master grating and the copy hologram being rigidly secured to each other and face each other by their recording layers (cf., for example, V. A. Vanin, L. N. Vagin, Interference Copying of Microholograms, Zhurnal nauchnoi i prikladnoi fotografiii i kinotekhniki, in Russian, 1975, vol. 20, issue No. 6, pp. 416-422).
This device makes use of the interference copying process and operates as follows.
A part of the collimated beam of coherent radiation, which is incident upon the hologram grating, diffracts thereon, whereas a part of this beam passes without deviation. These two beams interfere and produce, in the plane of the recording layer of the copy hologram, a standing wave image identical to the one in which the master hologram was recorded. If the collimated beam during copying is the same as one of the beams during the master hologram recording, the interference copying of the hologram grating takes place.
In this case the mechanical contact between the master and copy holograms is no longer used. This helps preserve the master hologram and the copies are more accurate. With the proper photochemical treatment, the copy hologram is very close to the master hologram. But this device is deficient in that copying can only be performed within the limits of the collimating system.